The present invention relates to an anti-lock brake system for automotive vehicles including a master brake cylinder, such as a tandem master cylinder, and a vacuum brake power booster which is inserted ahead of the master cylinder and is operable by means of an input member. The primary piston of the tandem master cylinder is of two part construction and includes an external piston and an internal piston guided within the external piston. A movable wall separating a vacuum chamber and a working chamber in the vacuum brake power booster is adapted to move independently of the input member and is in driving engagement with the external piston. Means are provided which, on failure of the primary pressure chamber of the tandem master cylinder, provide a force effective at the input member which is dependent on the inlet force and is opposed to the inlet force.
A similar actuating unit is described in German patent application P 39 19 216.4. In order that the pressure proportional to the inlet force can be developed in the secondary pressure chamber in the event of failure of the primary pressure chamber, a reaction device is arranged in the primary pressure chamber so that a boosting force proportional to the inlet force is transmitted onto the secondary piston. The reaction device is formed by an elastic reaction plate which is arranged in the force-transmitting sleeve. The plate is in operative engagement with the external piston by way of the force-transmitting sleeve and directly with the internal piston. An opening mechanism of a first central valve is supported on the plate. The central valve is allocated to the primary pressure chamber. Particularly disadvantageous characteristics of these devices are the additional axial overall length that is required for the installation of the reaction device and the complicated length dimensions for mounting purposes. The manufacturing expenses are accordingly excessive.
Therefore, it is an object of the present invention to substantially eliminate the above-mentioned disadvantages and to improve upon an actuating unit of the type described to accomplish shortening of the overall axial length of the actuating unit, while reducing the manufacturing costs.